Emergence of dengue in the tribal pockets of Nilgiris district,Tamil Nadu

Introduction

Dengue is the most rapidly spreading mosquito transmitted infection in the world and has been identified as a re-emerging arboviral disease in Southeast Asia. ¹ The disease poses a threat to more than 1.8 billion people in tropical and subtropical regions, affecting about 100 million people every year. ² There is no vaccine nor specific antidote available; the only strategic option is vector control to reduce viral transmission. India is one of seven identified countries annually reporting incidences of DF/DHF outbreaks and is a major hyperendemic niche for dengue infection, with more and more new areas being implicated. ³ Southern India has been signalled as a warning of future epidemics. Dengue has become an emerging serious public health problem in Tamil Nadu. ⁴ Retrospective analysis has showed a noticeable increase during the cooler, the monsoon and post-monsoon months. ⁵ However, dengue is also reported during the rainy season ⁶ and a seasonal shift seems to have taken place. ⁷

Reported information on the incidence of dengue among tribal populations is negligible even though the vector, Aedes albopictus, is present in the whole of the Western Ghats. The Indian tribal population constitutes 8.2% of the country’s total. ⁸ Dengue was first reported in Coimbatore in 1998. ⁹ No information concerning the epidemiology of dengue in the Nilgiris district of Tamil Nadu State of the Western Ghats is documented to date.

Methodology

The Nilgiris (meaning Blue Hills) are situated at an elevation of 900–2636 m above sea level. Its topography is rolling and steep and the cultivable land falls under the slopes. It has an area of 2452.50 km² with a population of 762,000. ¹⁰ Dams have been constructed where ever possible to exploit natural springs. During summer the temperature is in the range of 10–25℃ and during winter the range is 2–21℃.

Patients with suspected dengue categorised by the World Health Organization ² were submitted to blood sampling between July 2014 and February 2015 at the onset of their illness. Serum was centrifuged at 4℃ and stored at −70℃ for later analysis. All samples were individually labelled at four different collection points. Samples were subjected to NS1 (antigen), IgM and IgG antibody analysis (ELISA kit-PanBio) as per manufacturer’s protocol. Our study was approved by the centre’s ethics committee.

Results

In all, 182 samples were collected and screened for dengue and 7.14 % were found positive (Table 1). Dengue positives were distributed as shown (Tables 2–4). Most were reported during February 2015 (11.11%). No positive cases were recorded during the month of December 2014 (Table 5).

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Table 1.

Area-wise dengue-positive reported from Nilgiris District—July 2014 to February 2015.

S. no.	Tribe	Total serum (n)	+ve (n)	%
1	Kotagiri	96	8	8.33
2	Coonoor	25	0	0
3	Ooty	32	3	9.37
4	Gudalur	29	2	6.89
Grand total		182	13	7.14

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Table 2.

Tribe-wise dengue-positive reported from Nilgiris District—July 2014 to February 2015.

S. no.	Tribe	Total serum (n)	+ve (n)	%
1	Irula	89	7	7.86
2	Kurumba	28	1	3.57
3	Kota	15	3	20
4	Toda	23	1	4.34
5	Kattu Naikar	1	0	0
6	Paniyar	24	1	4.1
7	SC community	2	0	0
Grand total		182	13	7.14

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Table 3.

Age-wise dengue-positive reported from Nilgiris District—July 2014 to February 2015.

S. no.	Tribe	Total serum (n)	+ve (n)	Age group (%)
				21–30	31–40	41–50	51–60	61–70	71–80
1	Kotagiri	96	8	3 (3)	1 (1)	1 (1)	2 (2)	0	1 (1)
2	Coonoor	25	0	0	0	0	0	0	0
3	Ooty	32	3	0	2 (6.2)	0	1 (3.1)	0	0
4	Gudalur	29	2	1 (3.4)	0	1 (3.4)	0	0	0
Grand total		182	13	4 (2.1)	3 (1.6)	2 (1)	3 (1.6)	0	1 (0.5)

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Table 4.

Sex-wise dengue-positive reported from Nilgiris District—July 2014 to February 2015.

S. no.	Tribe	Total serum (n)	+ve (n)	Sex
				Male	Female
1	Kotagiri	96	8	1	7
2	Coonoor	25	0	0	0
3	Ooty	32	3	0	3
4	Gudalur	29	2	0	2
Grand total		182	13	1	12

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Table 5.

Month-wise dengue-positive reported from Nilgiris District—July 2014 to February 2015.

S. no.	Month of visit	Total serum (n)	+ve (n)	%
1	July/August 2014	53	5	9.4
2	October 2014	75	5	6.6
3	December 2014	27	0	0
4	February 2015	27	3	11.11
Total		182	13	7.14

Discussion

Hitherto unsurveyed areas such as the Nilgiris, previously assumed to be non-endemic, are now seeing dengue virus activity to varying degrees. Surveillance is therefore essential in these areas. An epidemiological investigation of the outbreak in the Coimbatore District, situated in the eastern slopes of the Nilgiris during 1998, 2003 and 2010 showed dengue-positive cases from some isolated villages. ⁸ Studies among semi-nomadic populations from the fringes of the Nilgiris during 2011 showed IgM positives. These people occupy the lower hills and trade with the people in the plains, where they come into contact with the vector and the virus. Others graze large herds of buffalo which may expose them to the dengue vector. All age groups have been found to be affected by dengue.

Our study has identified dengue as an emerging arboviral infection in this hilly district of Tamil Nadu and exposes the looming risk of its spread to neighbouring areas.

In the Nilgiris, there is a reduction of forested areas, owing to replacement by cash crops. Human settlements containing dengue vectors thus move into the rural environment. A coordinated, consistent, integrated vector management approach is needed to control dengue in these highly sensitive areas, especially where populations have little or no immunity to the disease.